Multi-zone data converters

ABSTRACT

Aspects of a method and system for data converters having a transfer function with multiple operating zones. In some embodiments, an operating zone of the multiple operating zones is characterized by more stringent performance criteria than the other operating zones. Thus, such data converters may receive an input signal and generate an output signal from the input signal per the transfer function and the more stringent performance criteria in the appropriate operating zone.

CLAIM OF PRIORITY

This patent application is a continuation of U.S. patent applicationSer. No. 15/364,577, filed Nov. 30, 2016, which is a continuation ofU.S. patent application Ser. No. 14/925,040, filed Oct. 28, 2015, nowU.S. Pat. No. 9,543,973, which is a continuation of U.S. patentapplication Ser. No. 14/623,743, filed Feb. 17, 2015, now U.S. Pat. No.9,178,526, which is a continuation of U.S. patent application Ser. No.14/246,576, filed Apr. 7, 2014, now U.S. Pat. No. 8,981,982, whichclaims priority to and claims benefit from the U.S. Provisional PatentApplication Ser. No. 61/808,855, filed on Apr. 5, 2013. The above statedapplications are hereby incorporated herein by reference in theirentirety.

TECHNICAL FIELD

Certain embodiments of the invention relate to electronic circuits. Morespecifically, certain embodiments of the invention relate to methods andsystems for multi-zone data converters.

BACKGROUND

Existing data converters can be expensive and power hungry. Furtherlimitations and disadvantages of conventional and traditional approacheswill become apparent to one of skill in the art, through comparison ofsuch systems with some aspects of the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method is provided for multi-zone data converters,substantially as shown in and/or described in connection with at leastone of the figures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates an example electronic system that may be operable toperform digital-to-analog and analog-to-digital conversions.

FIGS. 2A and 2B are diagrams illustrating transfer characteristics ofexample multi-zone analog-to-digital converters.

FIGS. 3A and 3B are diagrams illustrating transfer characteristics ofexample multi-zone analog-to-digital converters.

FIG. 4 depicts an example multi-zone flash ADC in accordance withaspects of this disclosure.

FIG. 5 depicts an example multi-zone DAC in accordance with aspects ofthis disclosure.

FIG. 6 is a flow chart illustrating an example process fordesigning/configuring a multi-zone data converter.

DETAILED DESCRIPTION

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. As another example,“x, y, and/or z” means any element of the seven-element set {(x), (y),(z), (x, y), (x, z), (y, z), (x, y, z)}. As utilized herein, the term“exemplary” means serving as a non-limiting example, instance, orillustration. As utilized herein, the terms “e.g.,” and “for example”set off lists of one or more non-limiting examples, instances, orillustrations. As utilized herein, circuitry is “operable” to perform afunction whenever the circuitry comprises the necessary hardware andcode (if any is necessary) to perform the function, regardless ofwhether performance of the function is disabled, or not enabled, by someuser-configurable setting.

Aspects of the invention include data converters customized for use insystems, such as communication systems, where much of the dynamic rangeof the data converters is present only to avoid clipping in the event ofinfrequent peaks (e.g., resulting from blocker signals). Because thesignals whose amplitudes range into the higher digital codes aretypically unwanted blocker signals, larger amounts of quantization noisemay be tolerable at such signal levels. Accordingly, the transferfunction of the data converters may be segmented into multiple zoneswith different zones having different characteristics (integralnonlinearity, differential nonlinearity, gain offset, and/or the like).For example, various components (e.g., comparators, resistors,capacitors, current sources, amplifiers, and/or the like) may havedifferent design constraints (e.g., size, power consumption, speed,impedance value matching in differential and/or switched capacitorcircuits, etc.) based on which bits of digital output code they areresponsible for generating.

FIG. 1 illustrates an example electronic system 100 that may be operableto perform analog-to-digital conversion and/or digital-to-analogconversion. The electronic system 100 may comprise suitable circuitry,interfaces, logic and/or code for implementing various aspects of thepresent disclosure. In this regard, the electronic system 100 may beconfigured to support performing, executing or running variousoperations, functions, applications and/or services. For example, theelectronic system 100 may be used for executing computer programs,playing video and/or audio content, gaming, communication applicationsor services (e.g., Internet access/browsing, email, text messaging,chatting and/or voice calling services), and/or networking services(e.g., WiFi hotspot, Bluetooth piconet, and/or active 3G/femtocell datachannels).

In some instances, the electronic system 100 may be configured to enableand/or support communication of data. In this regard, the electronicsystem 100 may communicate with other systems (local or remote), such asduring executing, running, and/or performing of operations, functions,applications and/or services supported by the electronic system 100. Forexample, the electronic system 100 may be configured to support (e.g.,using suitable dedicated communication components or subsystems) use ofwired and/or wireless connections/interfaces, which may be configured inaccordance with one or more supported wireless and/or wired protocols orstandards, to facilitate transmission and/or reception of signals(carrying data) to and/or from the electronic system 100. In thisregard, the electronic system 100 may be operable to process transmittedor received signals in accordance with applicable wired or wirelessprotocols.

Examples of wireless protocols or standards that may be supported and/orused by the communication subsystem 250 may comprise wireless personalarea network (WPAN) protocols, such as Bluetooth (IEEE 802.15); nearfield communication (NFC) standards; wireless local area network (WLAN)protocols, such as WiFi (IEEE 802.11); cellular standards, such as2G/2G+(e.g., GSM/GPRS/EDGE, and IS-95 or cdmaOne) and/or 2G/2G+(e.g.,CDMA2000, UMTS, and HSPA); 4G standards, such as WiMAX (IEEE 802.16) andLTE; Ultra-Wideband (UWB), direct broadcast satellite (DBS), microwavebackhaul, and/or the like. Examples of wired protocols and/or interfacesthat may be supported and/or used by the communication subsystem 250comprise Ethernet (IEEE 802.2), Fiber Distributed Data Interface (FDDI),Integrated Services Digital Network (ISDN), cable (DOCSIS) and UniversalSerial Bus (USB) based interfaces. Examples of signal processingoperations that may be performed by the electronic system 100 comprise,for example, filtering, amplification, analog-to-digital conversionand/or digital-to-analog conversion, up-conversion/down-conversion ofbaseband signals, encoding/decoding, encryption/decryption, and/ormodulation/demodulation.

In some instances, the electronic system 100 may be configured to enableor support input/output operations, such as to allow user interactionsthat may control services provided by the electronic system 100. In thisregard, the electronic system 100 may comprise components or subsystemsfor enabling interactions with a user (e.g., end-user or installer), soas to obtain user input and/or to provide user output.

The electronic system 100 may be a stationary system (i.e. beinginstalled at, and/or configured for use only in particular location). Inother instances, however, the electronic system 100 may be a mobiledevice—i.e., intended for use on the move and/or at different locations.In this regard, the electronic system 100 may be designed and/orconfigured (e.g., as handheld device) to allow for ease of movement,such as to allow it to be readily moved while being held by the user asthe user moves, and the electronic system 100 may be configured toperform at least some of the operations, functions, applications and/orservices supported on the move.

Examples of electronic systems may comprise handheld electronic devices(e.g., cellular phones, smartphones, or tablets), personal computers(e.g., laptops or desktops), servers, dedicated multimedia devices(e.g., televisions, game consoles, or portable media players), DBSoutdoor units, DBS indoor units (e.g., “set-top boxes”), microwavebackhaul outdoor units, microwave backhaul indoor units, or othersimilar receiver systems, and the like. The disclosure, however, is notlimited to any particular type of electronic system.

In operation, the electronic system 100 may be operable to performvarious operations, functions, applications and/or services. In thisregard, in some instances, performing the various, operations,functions, application or services supported by the electronic system100 may entail performing various processing operations on data handledby the electronic system 100. For example, communication of data,whether over wired or wireless interfaces, may typically comprisetransmitting and/or receiving analog signals that are communicated overwireless and/or wired connections. In this regard, typically analogradio frequency (RF) signals may be used to carry data (e.g., content),which may be embedded into the analog signals using analog or digitalmodulation schemes. For analog communications, data is transferred usingcontinuously varying analog signals, and for digital communications, theanalog signals are used to transfer discrete messages in accordance witha particular digitalization scheme.

Accordingly, handling of digital communications may typically requireperforming, among other things, digital-to-analog conversion (e.g., viaa digital-to-analog converter (DAC) 110) at the transmitting end andanalog-to-digital conversion (e.g., via a analog-to-digital converter(ADC) 120) at the receiving end. In this regard, the DAC 110 maycomprise circuitry, interfaces, logic and/or code for performingdigital-to-analog conversions and may implemented using various types ofDACs. For example, the DAC 110 may be implemented using a pulse-widthmodulator DAC, an oversampling DAC, interpolating DAC, a delta-sigmaDAC, binary-weight DAC, and R-2R ladder DAC, a successive-approximationDAC, a cyclic DAC, a thermometer-coded DAC, and/or a hybrid of one ormore of the previous DACs. Similarly, the ADC 120 may comprisecircuitry, interfaces, logic and/or code for performinganalog-to-digital conversions and may be implemented using various typesof ADCs. For example, the ADC 120 may be implemented using adirect-conversion ADC, a flash ADC, a successive-approximation ADC, aramp-compare ADC, a Wilkinson ADC, an integrating ADC, a dual-slope ADC,a multi-slope ADC, a delta-encoded ADC, a counter-ramp ADC, a pipelineADC, a subranging quantizer ADC, a sigma-delta ADC, a delta-sigma ADC, atime-interleaved ADC, etc.

In some electrical system 100, suitable performance of the electricalsystem 100 (e.g., measured in terms of signal to noise ratio, symbolerror rate, packet error rate, bit error rate, and/or some other metric)may require, or be more dependent upon, the data converter(s) (e.g., DAC110 and/or ADC 120) maintaining a high level of performance (measured interms of digital nonlinearity (DNL), integral nonlinearity (INL), gainoffset, and/or the like) in one or more ranges or zones of operation,but may not require, or may be less dependent upon, the dataconverter(s) maintaining a high level of performance in one or moreother ranges or zones of operation. In such situations, variousperformance criteria of the DAC 110 and/or the ADC 120 may be relaxed inoperations zones where lower performance has a lesser impact on theperformance of the electrical system 100. In this manner, the electricalsystem 100 may be able to achieve a level of performance typicallyassociated with use of high-end, high-cost data converters when usinglower-cost data converters that provide better performance in zones ofoperation having greater influence on the performance of the electricalsystem 100, but lesser performance in zones of operating having a lesserinfluence on the performance of the electrical system 100.

To this end, a transfer function of an example ADC 120 is shown in FIG.2A. As shown, a first zone Zone_1 may range from analog input value of 0to V1 (in the example shown V1=2×VLSB, and VLSB is the voltagecorresponding to one least significant bit), a second zone Zone_2 mayrange from an analog input value of V1 to V2 (V2=5*VLSB in the exampleshown), and a third zone Zone_3 may range from an analog input value ofV2 to +VFS (full-scale)(where VFS=7*VLSB in the example shown). Forexample, the first zone Zone_1 may be characterized by first integralnonlinearity INL_1, the second zone Zone_2 may be characterized bysecond integral nonlinearity INL_2, and the third zone Zone_3 may becharacterized by a third integral nonlinearity INL_3. The secondintegral nonlinearity INL_2 may be less than the first integralnonlinearity and may also be less than the third integral nonlinearity(i.e., INL_2<INL_1 and INL_2<INL_3).

Referring now to FIG. 2B, another transfer function for an example ADC120 is shown. As shown, the transfer function may include a first zoneZone_1 that ranges from an analog input value of −VFS to −V1, a secondzone Zone_2 that ranges from an analog input value of −V1 to +V1, and athird zone Zone_3 that ranges from an analog input value of +V1 to +VFS.Similar, to the transfer function of FIG. 2A, performance criteria(e.g., specified in terms of digital nonlinearity (DNL), integralnonlinearity (INL), gain offset, and/or the like) may be relaxed in theouter zones Zone_1 and Zone_3 and more stringent in the central zoneZone_2.

Thus, as shown in FIGS. 2A and 2B, an example ADC 120 may be implementedsuch that the second zone Zone_2 achieves better performance (e.g.,measured in terms of digital nonlinearity (DNL), integral nonlinearity(INL), gain offset, and/or the like) than the first zone Zone_1 and thethird zone Zone_3. If performance of the second zone Zone_2 is moreimportant to the performance of the electrical system 100 than theperformance of the first zone Zone_1 and the third zone Zone_3, then theelectrical system 100 may be able to achieve an operating performancewith such an ADC 120 that typically would require a higher-cost ADC thatperforms as well in all zones as ADC 120 does in the second zone Zone_2.

FIGS. 3A and 3B illustrate example DAC transfer functions which exhibita multi-zone concept similar to the ADC transfer functions of FIGS. 2Aand 2B. In particular, a transfer function of an example DAC block 120is shown in FIG. 3A. As shown, a first zone Zone_1 may range from adigital input code of 000 to 010, a second zone Zone_2 may range from adigital input code 010 to 101, and a third zone Zone_3 may range from adigital input code of 101 to 111. The first zone Zone_1 may becharacterized by first digital nonlinearity DNL_1, the second zoneZone_2 may be characterized by second digital nonlinearity DNL_2, andthe third zone Zone_3 may be characterized by third digital nonlinearityDNL_3. The second digital nonlinearity DNL_2 may be less than the firstdigital nonlinearity DNL_1 and may be also less than the third digitalnonlinearity (i.e., DNL_2<DNL_1 and DNL_2<DNL_3).

Referring now to FIG. 3B, another transfer function for an example DAC110 is shown. As shown, the transfer function may include a first zoneZone_1 that ranges from a digital input code of 000 to 010, a secondzone Zone_2 that ranges from a digital input code of 010 to 100, and athird zone Zone_3 that ranges from a digital input code of 100 to 110.Similar to the transfer function of FIG. 3A, performance criteria (e.g.,specified in terms of digital nonlinearity (DNL), integral nonlinearity(INL), gain offset, and/or the like) may be relaxed in the outer zonesZone_1 and Zone_3 and more stringent in the central zone Zone_2.

Thus, as shown in FIGS. 3A and 3B, an example DAC 110 may be implementedsuch that the second zone Zone_2 provides better performance (e.g.,measured in terms of digital nonlinearity (DNL), integral nonlinearity(INL), gain offset, and/or the like) than the first zone Zone_1 and thethird zone Zone_3. If performance of the second zone Zone_2 is moreimportant to the performance of the electrical system 100 thanperformance of the first zone Zone_1 and the third zone Zone_3, then theelectrical system 100 may be able to achieve an operating performancewith such an DAC 110 that typically would require a higher-cost DAC thatperforms as well in all zones as DAC 110 does in the second zone Zone_2.

The boundaries between zones (e.g., how low to set the split betweenZone_1 and Zone_2 and/or how high to set the split between Zone_2 andZone_3 in the previous examples) may be determined based oncharacteristics of signals to be processed by the data converters 110,120. Performance criteria (e.g., INL, DNL, gain offset, and/or the like)for each zone of any given DAC or ADC may similarly be determined basedon characteristics of signals to be processed by DAC or ADC. Fordigitizing received communications signals, for example, highestperformance (e.g., relatively low INL, relatively low DNL, andrelatively low gain offset) may be required of the mid-range digitalinput/output codes or analog input/output voltage values while lowerperformance (e.g., relatively high INL, relatively high DNL, and/orrelatively high gain offset) may be tolerated for low-range andhigh-range digital input/output codes or analog input/output voltagevalues. Thus, cost-savings may be achieved throughsimpler/smaller/faster/etc. circuitry used for processing the low-rangeand high-range signals, examples of which are described below withreference to FIGS. 4 and 5.

FIG. 4 depicts an example multi-zone flash ADC in accordance withaspects of this disclosure. The flash ADC 400 shown in FIG. 4 is anexample implementation of ADC 120 described above. In operation, eachcomparator 402 x (for 1×6) compares the input analog voltage to arespective reference voltage. The reference voltages increase as Xincreases. In an example implementation, comparators 402 ₁ and 402 ₂correspond to Zone_1, comparators 402 ₃ and 402 ₄ correspond to Zone_2,and comparators 402 ₅ and 402 ₆ correspond to Zone_3. Accordingly,performance criteria for the comparators 402 ₁, 402 ₂, 402 ₅, and 402 ₆(e.g., gain, resolution, input offset, propagation delay, slew rate,and/or the like) may be relaxed as compared to the performance criteriafor comparators 402 ₃ and 402 ₄. This may enable the comparators 402 ₁,402 ₂, 402 ₅, and 402 ₆ to be smaller, consume less power, have relaxedlayout constraints, and/or the like as compared to the comparators 402 ₃and 402 ₄.

FIG. 5 depicts an example multi-zone DAC in accordance with aspects ofthis disclosure. The example DAC 500 is an example implementation of theDAC 110 described above. In operation, the value of the digital inputvalue determines which of the switches 504 ₁-504 ₇ are closed. For 000all switches 504 ₁-504 ₇ are open, for 001 switch 504 ₁ is closed andthe rest are open, for 010 switches 504 ₁ and 504 ₂ are closed and therest are open, and so forth. In an example implementation currentsources 502 ₁-502 ₄ and switches 504 ₁-504 ₄ correspond to a first zonethat requires high performance and current sources 502 ₅-502 ₇ andswitches 504 ₅-504 ₇ correspond to a second zone which can toleratedecreased performance. In such an implementation, the current sources502 ₅-502 ₆ may be lower cost (smaller, lower power consumption, lessaccurate matching, etc.) than current sources 502 ₁-502 ₄ and switches504 ₅-504 ₇ may be lower cost than switches 504 ₁-504 ₄ (e.g., switches504 ₁-504 ₄ may be transmission gates and switches 504 ₅-504 ₇ may besingle transistors).

FIG. 6 is a flow chart illustrating an example process fordesigning/configuring a multi-zone data converter. In block 604, afterstart block 602, a signal to be converted is characterized. This may bedone, for example, based on standard/protocols that govern the signaland/or based on characterization of the signal using test equipment. Foranalog-to-digital conversion, the characterization may comprise, forexample, generating a probability vs. analog input value graph or table.For digital-to-analog conversion, the characterization may comprise, forexample, generating a probability vs. digital input value graph ortable. In block 606, the number of operation zones of the converter, andboundaries between those zones, is determined. For example, usingdistinct transitions between areas of relatively high probability andareas of low probability may be mapped to boundaries between zones. Inblock 608, the converter is configured/designed to have the number ofzones and boundaries between zones determined in block 606. This mayinclude, for example, implementing relatively high performancecomponents (e.g., comparators, current sources, switches, etc.) for thehigh-probability zone(s) and relatively low performance components forthe low-probability zone(s). In block 610, the converter is used forconverting (A to D or D to A) the signal.

Other embodiments of the invention may provide a non-transitory computerreadable medium and/or storage medium, and/or a non-transitory machinereadable medium and/or storage medium, having stored thereon, a machinecode and/or a computer program having at least one code sectionexecutable by a machine and/or a computer, thereby causing the machineand/or computer to perform the processes as described herein.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputing system, or in a distributed fashion where different elementsare spread across several interconnected computing systems. Any kind ofcomputing system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computing system with a program orother code that, when being loaded and executed, controls the computingsystem such that it carries out the methods described herein. Anothertypical implementation may comprise an application specific integratedcircuit or chip.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

What is claimed is:
 1. A multi-zone digital-to-analog converter,comprising a digital input configured to receive a digital input signal;an analog output configured to output an analog output signalrepresentative of the digital input signal; a first plurality of currentsources associated with a first voltage range of the analog outputsignal; a second plurality of current sources associated with a secondvoltage range of the analog output signal; and a plurality of switchesthat selectively couple the first plurality of current sources and thesecond plurality of current sources to the analog output in order toprovide the analog output with the analog output signal representativeof the digital input signal; wherein the first plurality of currentsources provide the first voltage range with one or more performancecharacteristics; wherein the second plurality of current sources providethe second voltage range with a corresponding one or more performancecharacteristics; and wherein the one or more first performancecharacteristics provided by the first plurality of current sources arehigher than the corresponding one or more second performancecharacteristics provided by the second plurality of current sources. 2.The multi-zone digital-to-analog converter of claim 1, furthercomprising a decoder configured to selectively close the plurality ofswitches based on the digital input signal.
 3. The multi-zonedigital-to-analog converter of claim 1, wherein the first voltage rangeis below the second voltage range.
 4. The multi-zone digital-to-analogconverter of claim 1, wherein the one or more performancecharacteristics of the first voltage range result in the secondplurality of current sources consuming less power than the firstplurality of current sources.
 5. The multi-zone digital-to-analogconverter of claim 1, wherein the one or more performancecharacteristics of the first voltage range result in the secondplurality of current sources being smaller than the first plurality ofcurrent sources.
 6. The multi-zone digital-to-analog converter of claim1, wherein the one or more performance characteristics of the firstvoltage range result in the second plurality of current sources havingless accurate matching than the first plurality of current sources. 7.The multi-zone digital-to-analog converter of claim 1, wherein: eachswitch, from the plurality of switches that is associated with a currentsource from the first plurality of current sources, comprisestransmission gate switch; and each switch, from the plurality ofswitches that is associated with a current source from the secondplurality of current sources, comprises a single transistor switch.
 8. Amethod of a multi-zone digital-to-analog converter, the methodcomprising: receiving a digital input signal with a digital input of themulti-zone digital-to-analog converter; selectively closing switchesfrom a plurality of switches of the multi-zone digital-to-analogconverter based on the digital input signal received by the digitalinput to selectively couple current sources from a first plurality ofcurrent sources and a second plurality of current sources to an analogoutput of the multi-zone digital-to-analog converter; and outputting ananalog output signal representative of the digital input signal based onthe current sources from the first plurality of current sources and thesecond plurality of current sources coupled to the analog output,wherein the first plurality of current sources provide a first voltagerange of the analog output with one or more performance characteristics,the second plurality of current sources provide a second voltage rangeof the analog output with a corresponding one or more performancecharacteristics; and the one or more first performance characteristicsprovided by the first plurality of current sources are higher than thecorresponding one or more second performance characteristics provided bythe second plurality of current sources.
 9. The method of claim 8,wherein said selectively closing comprises a decoder of the multi-zonedigital-to-analog converter selectively closing the switches from theplurality of switches based on the digital input signal.
 10. The methodof claim 8, wherein said selectively closing comprises selectivelycoupling, to the analog output, the current sources from the firstplurality of current sources and the second plurality of current sourcessuch that the first voltage range is below the second voltage range. 11.The method of claim 8, further comprising consuming less power when theanalog output signal is in the second voltage range than when the analogoutput signal is in the first voltage range.
 12. The method of claim 8,wherein said selectively closing comprises selectively coupling, to theanalog output, second current sources from the second plurality ofcurrent sources that are smaller than first current sources from thefirst plurality of current sources when generating the analog outputsignal in the second voltage range.
 13. The method of claim 8, whereinsaid selectively closing comprises selectively coupling, to the analogoutput, second current sources from the second plurality of currentsources that have less accurate matching than first current sources fromthe first plurality of current sources when generating the analog outputsignal in the second voltage range.
 14. The method of claim 8, whereinsaid selectively closing comprises: selectively closing transmissiongate switches from the plurality of switches that are associated withthe first plurality of current sources; and selectively closing singletransistor switches from the plurality of switches that are associatedwith the second plurality of current sources.